You could just pull the 20A breaker and put in a 15A breaker. That's the easy, code compliant solution. Also 14AWG won't set on fire until it's got 50-60A passing through it. Heat and age come in to play if it were to continuouldy and frequently see too much current the insulation could wear down over time and expose you to some risk. When in doubt, for all things safety related, I say take action and make them right. when you adopt the mindset of "it'll be fine" and cut a corner, you're gambling. If you gamble enough, you're a gambler. You will lose one day, ask any gambler. Will it cause a fire? I highly doubt it. doubt isn't certainty though... good luck, and change that breaker, ya hack!!! ;-)